1. Field of the Invention
This invention relates to a method for preventing agglomeration of a colloidal silica by trialkylsilylating, in part, the silanol groups of silica particles contained in the colloidal silica, and to the composition for a silicon wafer polishing containing a colloidal silica containing partially trialkylsilylated silica particles.
2. Description of the Prior Art
Surfaces of silicon wafers used as substrates in the production of semiconductor devices such as integrated circuits are mirror-polished and then subjected to the formation of circuits.
There have heretofore been proposed various types of polishing compositions for polishing the surface of silicon wafers. For example, in the specification of U.S. Pat. No. 3,170,273, a silica sol having a silica concentration of 2 to 50% by weight and a silica gel having a silica concentration of 2 to 100% by weight are disclosed as polishing compositions. The specification of U.S. Pat. No. 3,328,141 discloses that the polishing speed is increased by mixing these polishing compositions with an alkaline compound to adjust the pH value to 10.5 to 12.5.
Furthermore, the specification of U.S. Pat. No. 4,664,679 discloses that silicic anhydride obtained by a dry method is surface treated with silane-based or other coupling agents to prepare a stable silica aqueous dispersion composition, which is applied as a polishing agent.
For polishing silicon wafers, colloidal silica is widely used for its small particle sizes over aqueous dispersion compositions of silicic anhydride.
However, colloidal silica, because it has chemically active silanol groups in a high density on the surface of silica particles, tends to undergo gelation during drying or, even as is, produce coarse silica particle agglomerations. When polishing slurry is contaminated with a gelled matter from the slurry supply or agglomerations which produce coarse silica particles are present, the polishing slurry tends to cause scratches or latent flaws on the wafer surface during polishing of the silicon wafers.
These scratches and latent flaws present on the silicon wafers are not preferable since they lead to uneven electrical characteristics of circuits when the circuit patterns are formed on the surface of the silicon wafers using oxide film formation, etching, and doping techniques.
Therefore,the polishing slurry must be strictly controlled so as not to be contaminated with dried gelation products or coarse silica particle agglomerations, but there is no method for checking the polishing slurry whether or not it is contaminated with dried gelation products or agglomerations, which can only be determined from the polished silicon wafers.
On the other hand, in an aqueous dispersion composition of silicic anhydride, even when dried, the gelation products are not high in mechanical strength and can easily be broken, but stability of an aqueous dispersion composition of silicic anhydride has been inferior to that of colloidal silica.
With a view to eliminate the above prior art problems, it is a primary object of the present invention to provide a polishing composition for silicon wafers, in which silica particles in a colloidal silica is modified so that even when the colloidal silica is dried and gelled, it can easily be repulverized by a weak mechanical action, and that it has less tendency to generate coarse silica particles, to obtain the polishing composition containing such a modified colloidal silica, thereby preventing generation of scratches or latent flaws on the surface of silicon wafers during polishing.